In many communication system applications, it is important to provide alignment between particular timing intervals across multiple communication channels. One such application is in aligning timing intervals associated with different upstream data transmission channels in a cable television system, also commonly referred to as a community access television (CATV) system, as will be described below.
In a CATV system, cable modems (CMs) communicate with a cable-modem termination system (CMTS) over a CATV network. The cable-industry-backed standard that specifies how a CM and CMTS communicate with each other is called DOCSIS (Data Over Cable Service Interface Specification), and is described in the DOCSIS specification documents, which are incorporated by reference herein.
A DOCSIS-compliant network is frequency-division multiplexed (FDM) into one or more downstream (CMTS-to-CM) and one or more upstream (CM-to-CMTS) channels, with each channel corresponding to one FDM frequency. Data sent on a downstream channel is physically broadcast to all CMs that can receive that channel, and each CM picks off the packets that are intended for that CM. During normal operation a CM listens on only one downstream channel and transmits on only one upstream channel. CMs cannot directly communicate with each other. All data sent on a given upstream channel goes to the CMTS serving that channel. A typical CMTS serves between about 500 and 2,000 CMs.
With regard to the issue of alignment of timing intervals, the DOCSIS standard requires that initial-maintenance intervals (IMIs) in certain sets of upstream channels be aligned. DOCSIS does not disclose or suggest any particular techniques by which a CMTS might provide such alignment.
A need therefore exists for techniques for aligning timing intervals in cable-modem upstream channels in a CATV system, as well as in other communication system applications.